Enable all CoroutineExecutorService tests in CI

[BenHenning]

With the recent introduction of CoroutineExecutorService in #1764, tests were also added to verify that the service follows Java's ExecutorService contract correctly in a way that's interoperable with Oppia's test dispatchers. While the tests are fairly thorough, they were exceptionally difficult to build due to needing to coordinate between custom test dispatchers, Oppia's centralized test dispatchers, and real dispatchers to test blocking behaviors (where coroutines generally don't like blocking scenarios). Most tests are fairly stable, but a few are still flaky and will require substantial investigation to stabilize. This issue will stay open until all tests in this suite can pass with at least 100 runs in the CI environment (since it's slower than most developer machines, and these tests can be sensitive to CPU speeds).

Note also that this is 'nice-to-have' since we don't expect any code iteration to occur in CoroutineExecutorService after initial introduction.

[BenHenning]

We've learned a lot since this was originally filed. #3715 is actually a better approach as it has lots of downstream benefits (namely allowing us to coordinate execution with third-party Java libraries that allow for executor customization, like OkHttp, Retrofit, and Glide). Trying to make a coroutine executor service work was very difficult, especially in testing. I'd much rather abandon this approach in favor of the coordinated executors.

[github-actions]

The issue is reopened because of the following unresolved TODOs:

oppia-android/testing/src/test/java/org/oppia/android/testing/threading/CoroutineExecutorServiceTest.kt

Line 686 in 0ef7a46

@Ignore("Flaky test") // TODO(#1763): Remove & stabilize test.

oppia-android/testing/src/test/java/org/oppia/android/testing/threading/CoroutineExecutorServiceTest.kt

Line 983 in 0ef7a46

@Ignore("Flaky test") // TODO(#1763): Remove & stabilize test.

[BenHenning]

Ah, this issue can be closed with the removal of CoroutineExecutorService.